1. Field of the Invention
The present invention relates to a technique for mapping asynchronous fixed-length cells such as ATM cells, etc. onto an existing synchronous digital interface.
2. Description of the Related Art
Synchronous digital private lines which are conventionally connected to an enterprise have been increasingly shifted to ATM (Asynchronous Transfer Mode) lines by using TDMs (Time Division Multiplexers), etc.
In such a case, a TDM arranged in an enterprise is replaced with an ATM switch, to which a PBX (Private Branch exchange), an ATM terminal, etc. inside the enterprise are connected.
Since communications are asynchronously made and communications channels are not always busy with the ATM system, the effect of multiplexing is significant and multimedia data, etc. can be efficiently transmitted.
An ATM line has the transmission rate of at least 155 Mbps. If one certain enterprise attempts to newly adopt ATM lines instead of synchronous digital private lines in a current state, it requires a lot more cost.
Therefore, there is a conventional technique for mapping ATM cells onto a synchronous digital interface format as a technique for promoting the shift from a synchronous digital communication to an ATM communication. As typical techniques, the techniques stipulated by the ITU-T G.804, the ATM Forum AF-PHY-0016.000, the UNI version 3.1, etc. are known.
Use of such techniques allows a TDM, etc. arranged in an enterprise to be replaced with an ATM switch without changing an existing synchronous digital interface on a network side and an existing PBX and terminal equipment, etc. inside the enterprise.
More specifically, the ATM switches to which the above described techniques are applied are arranged at both ends of a private synchronous private line. By connecting existing PBXs and terminals in an enterprise to the ATM switches at the beginning of the shift, ATM cells are mapped onto a synchronous digital interface and transmitted by a communication between the ATM switches at both of the ends without user's awareness of the shift from the TDM devices, etc. to the ATM switch. At a later stage of the shift, ATM terminals, etc. are newly connected to the ATM switches, so that the user can smoothly and gradually shift the existing terminals, etc. to the ATM terminals, etc.
As a current contract form of the synchronous digital private line, not an entire bandwidth, but only a bandwidth according to an amount required by traffic can be contracted. In this case, a user will use a predetermined number of time slots among the time slots allocated to a synchronous digital private line via a TDM, etc.
With the above described conventional technique for mapping ATM cells onto a synchronous digital interface, however, the ATM cells are mapped onto an entire bandwidth (all time slots) of the physical line configuring the synchronous digital interface. Therefore, this mapping technique has the problem of being unable to be applied to the implementation in which only part of the bandwidth (some of the time slots) of the synchronous digital private line is used.